Don't forget autumn N

What are you going to do for supplementary feed this winter? Maize and palm kernel extract have been popular choices in recent times, and there are a host of other feed options available, including many different food industry by-products. Or, of course, you can grow your own.

When it comes to choosing a supplementary feed, there are a number of factors to consider: price; availability; ease of storage; nutritional value and palatability, to name just a few. Of these, price is often of most concern to farmers. Since yield will be driven by the amount of energy in the cow’s feed (as long as there are no nutritional imbalances or animal health issues to manage), the figure that matters is the cost per megajoule of metabolisable energy (MJ ME).

If you’re purchasing supplementary feed, it’s relatively easy to work out the cost per MJ ME. The important things to know are how much dry matter (DM) is in each tonne of the feed, the MJ ME per kg DM in the feed, and the percentage utilisation of the feed. You’ll also need to factor in the cost of feeding out (in most cases a good number to use would be 5 cents/kg DM).

If you’re going to use urea to grow your additional pasture so you don’t have to buy in supplementary feed – or at least, so you can buy less - then there are some additional variables to consider: the price of urea, and the pasture response to that urea.

To work out the cost of growing your own feed, follow these steps:

  • Calculate the on-the-ground cost of the nitrogen in the urea. If the cost of your urea, cartage and spreading is $700, that works out at $1.52 per kg of N ($700 ¸ 460 – the number of kg of N in a tonne of urea).
  • If you get a conservative 10:1 response to the nitrogen, then each kg DM grown will have cost 15.2 cents ($1.52 ¸ 10).
  • This feed will typically have 11 MJ ME/kg DM. This means each MJ ME will have cost 1.38 cents to grow (15.2 ¸ 11).
  • There is normally 20% wastage when grazing pasture, so we have 80% utilisation of the feed, which means the cost of each MJ ME down the cow’s throat is 1.73 cents (1.38 x 100 ¸ 80).

Of late, the price of nitrogen in urea has been trending downwards slightly, but there are indications that this will stop and that prices will creep upwards again. Nonetheless, it is likely that these rises will not be sufficient to upset the economics of urea use.

You might think that autumn responses to N would be lower than spring responses, given that soil temperatures are on their way down. What we actually see is a more even response over a longer time period (see Figure 1). In spring the flush of growth produces a rapid spike in grass production; in autumn the same level of production can be achieved, it just takes longer. Follow these guidelines for successful pasture response from n-rich urea:

  • Apply N to actively growing pastures. Soil temperatures should be above 5C otherwise growth will be severely compromised
  • Leave around 1200 kg DM cover on pastures after grazing; lower residuals will result in lower responses
  • Apply N at the recommended level of 30-50 kg N/ha (70-100 kg n-rich urea/ha)
  • Allow enough time for the N to have an effect; don’t graze or harvest too soon
  • When shutting up paddocks for silage, don’t apply potassium beforehand unless there is a growth-limiting deficiency.

Seasonal response to N

Figure 1: Seasonal response patterns to n-rich urea.

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